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I'm confused about how oxygen sensors work #5427312 05/12/20 12:22 PM
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OldEuroCarLover Offline OP
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Over the years I've replaced many o2 sensors in many cars. Often I'll replace the upstream sensors around the 100k-150k mile range as a maintenance item. The upstream replacement--typically done around the time of other items such as trans flush, ECT replacement, filters, spark plugs, etc--often results in slightly higher fuel mileage. (Stating all of that because it obviously doesn't directly implicate the replacement of the o2 sensors as the cause of improvement.)

Which leads to my question: do the downstream sensors ever matter for fuel injection calculation, or are they simply only there to indicate a bad catalyst? This has always been my understanding, and I've gotten up to around 250k miles without replacing downstream sensors.

Re: I'm confused about how oxygen sensors work [Re: OldEuroCarLover] #5427321 05/12/20 12:31 PM
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bdcardinal Offline
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The rear O2s are there only to monitor catalyst efficiency. For a while Ford would refer to them as Cat Monitors.

Edit, to expand I have read that some applications will take input from the cat monitors to try and change fuel trim to see if the cat is bad, but this is not common.

Last edited by bdcardinal; 05/12/20 12:31 PM.

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Re: I'm confused about how oxygen sensors work [Re: OldEuroCarLover] #5427324 05/12/20 12:32 PM
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The catalytic converter needs oxygen to work correctly. There is an incineration process going on in the CAT. The O2 sensor ensures that there is sufficient oxygen left over from the combustion process, whether by injection, or some other route, to support the needs of the CAT.
I can't see how it might impact fuel economy in any way though. It's not like a MAF sensor or anything that controls air or mixture PRIOR to combustion.

Re: I'm confused about how oxygen sensors work [Re: OldEuroCarLover] #5427325 05/12/20 12:32 PM
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Char Baby Offline
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The way that I understand it is that they just relay information to the computer.
The computer determines what is in spec or not. As sensors age due to too many heat cycles, they get "lazy" so to speak and the computer gives false readings...or doesn't know what to do. So it triggers a CEL and it's up to you or the tech to find out what is going on...if I said that right.

Last edited by Char Baby; 05/12/20 12:33 PM.

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Re: I'm confused about how oxygen sensors work [Re: JohnG] #5427337 05/12/20 12:44 PM
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thastinger Offline
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Originally Posted by JohnG
The catalytic converter needs oxygen to work correctly. There is an incineration process going on in the CAT. The O2 sensor ensures that there is sufficient oxygen left over from the combustion process, whether by injection, or some other route, to support the needs of the CAT.
I can't see how it might impact fuel economy in any way though. It's not like a MAF sensor or anything that controls air or mixture PRIOR to combustion.


O2 sensors control your short term fuel trim tables and data from them build your long term fuel tables.

Re: I'm confused about how oxygen sensors work [Re: bdcardinal] #5427355 05/12/20 01:08 PM
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supton Offline
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Originally Posted by bdcardinal
The rear O2s are there only to monitor catalyst efficiency. For a while Ford would refer to them as Cat Monitors.

Edit, to expand I have read that some applications will take input from the cat monitors to try and change fuel trim to see if the cat is bad, but this is not common.

That's kinda the problem here: what one OEM does might only apply to them. Some things are common enough to be a rule, others not so much. Have to hold rules of thumb with some level of skepticism. No sooner do you learn the rules that you learn of some exception...

Some time ago I got bored and hooked up Torq to look at the O2 sensors in my Camry. Rear O2 was dead. Not slow moving, I mean, dead--it was sitting at zero volts. I went on line, ordered the sensor. Replaced it a week later. Now I have no idea when it died, but it had a couple hundred miles and a bunch of restarts before I replaced it. And... it never set a CEL. shrug How's that work? (other than to my advantage!)


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Re: I'm confused about how oxygen sensors work [Re: OldEuroCarLover] #5427359 05/12/20 01:13 PM
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The rear O2 sensors in Dodge’s LX vehicles is mounted dead center in the cat, which is different from every other vehicle I ever had where they were actually mounted downstream of the cat. How that factors into the “used by PCM to adjust fuel trims” argument IDK, but it’s definitely different than what I’m used to.


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Re: I'm confused about how oxygen sensors work [Re: OldEuroCarLover] #5427375 05/12/20 01:26 PM
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CT8 Offline
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Basically O2 sensors work by generating a voltage between the atmospheric o2 and the exhaust gas 02.so the computer can adjust. The post converter sensor is a teltale to let the computer the pre cat sensor is functioning.


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Re: I'm confused about how oxygen sensors work [Re: bdcardinal] #5427379 05/12/20 01:30 PM
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OldEuroCarLover Offline OP
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Originally Posted by bdcardinal
The rear O2s are there only to monitor catalyst efficiency. For a while Ford would refer to them as Cat Monitors.

Edit, to expand I have read that some applications will take input from the cat monitors to try and change fuel trim to see if the cat is bad, but this is not common.


This is what I've seen in a few places, and has me confused.

I've always looked at the downstream sensors as just that: cat monitors. A code means the sensor or the cat is bad, and never anything else. But it's starting to seem like it's not always that simple.

Re: I'm confused about how oxygen sensors work [Re: The_Nuke] #5427381 05/12/20 01:31 PM
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Originally Posted by The_Nuke
The rear O2 sensors in Dodge’s LX vehicles is mounted dead center in the cat, which is different from every other vehicle I ever had where they were actually mounted downstream of the cat. How that factors into the “used by PCM to adjust fuel trims” argument IDK, but it’s definitely different than what I’m used to.



Chrysler is a weird exception

Some of them do use the rear post cat O2 to influence the front sensors decision making on fuel trims


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Re: I'm confused about how oxygen sensors work [Re: michaelluscher] #5427396 05/12/20 01:41 PM
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OldEuroCarLover Offline OP
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Originally Posted by michaelluscher
Originally Posted by The_Nuke
The rear O2 sensors in Dodge’s LX vehicles is mounted dead center in the cat, which is different from every other vehicle I ever had where they were actually mounted downstream of the cat. How that factors into the “used by PCM to adjust fuel trims” argument IDK, but it’s definitely different than what I’m used to.



Chrysler is a weird exception

Some of them do use the rear post cat O2 to influence the front sensors decision making on fuel trims


Is this strictly Chrysler? When did they start doing it? Do Mercedes or Fiat have a similar system?

Re: I'm confused about how oxygen sensors work [Re: OldEuroCarLover] #5427454 05/12/20 02:46 PM
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I've seen reference to the rear O2 sensors adjusting fuel trims on some BMW models.

Rear O2 Sensor Fuel Trim
The rear oxygen sensor, located after the catalyst, is used for fuel trim corrections on some
OBD-II vehicles. By virtue of its location, the rear sensor is generally protected from high
temperatures and much of the contamination that affects the front oxygen sensors. In addition,
the rear sensor sees exhaust gases that are equilibrated – they have already been converted
by the catalyst so that there is very little residual oxygen. This allows the rear sensor to
respond to much smaller changes in exhaust gas oxygen content. In turn, it then possible for
the rear sensor voltage to remain near the 0.45 volt switchpoint. This characteristic allows the
rear sensor to be used for fuel control. Under steady rpm and load conditions, the short term
fuel trim bias can be adjusted so that the rear sensor voltage is maintained near the 0.45 volt
switchpoint.


Here is a link to download the Baum Tools BMW 39 document:
BMW E39 Fuel Trims

Last edited by shanneba; 05/12/20 02:47 PM.

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Re: I'm confused about how oxygen sensors work [Re: The_Nuke] #5427517 05/12/20 04:45 PM
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eljefino Offline
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Originally Posted by The_Nuke
The rear O2 sensors in Dodge’s LX vehicles is mounted dead center in the cat, which is different from every other vehicle I ever had where they were actually mounted downstream of the cat. How that factors into the “used by PCM to adjust fuel trims” argument IDK, but it’s definitely different than what I’m used to.


You probably have a 3-way catalyist with the rear part doing NOx. Is there an air injection port system?

O2 sensors fail by getting "lazy", slower to switch. A correctly functioning catalytic converter is confirmed by a lazy rear o2 sensor reading, so replacing it with a "sharp" one can bring you peril in an older car.

Re: I'm confused about how oxygen sensors work [Re: The_Nuke] #5427578 05/12/20 06:06 PM
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bdcardinal Offline
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Originally Posted by The_Nuke
The rear O2 sensors in Dodge’s LX vehicles is mounted dead center in the cat, which is different from every other vehicle I ever had where they were actually mounted downstream of the cat. How that factors into the “used by PCM to adjust fuel trims” argument IDK, but it’s definitely different than what I’m used to.


I have seen some Ford and Mazda cats like that as well.

And not to add to the confusion, but I have seen some Mazdas where there are 3 sensors per bank. An A/F sensor pre-cat, a sensor in the cat, and one post cat.


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Re: I'm confused about how oxygen sensors work [Re: OldEuroCarLover] #5427591 05/12/20 06:25 PM
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hallstevenson Offline
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Originally Posted by OldEuroCarLover
Over the years I've replaced many o2 sensors in many cars. Often I'll replace the upstream sensors around the 100k-150k mile range as a maintenance item.

Truly curious, why replace them if they're still working ? It's not like when one fails it will leave you stranded.

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